Several 3-D modeling tools have been appeared on the web recently. These tools allow users to author and create 3-D models and then upload them into a database. This database of user-generated models can be published on platforms such as Virtual Earth 3-D which allow a user to visually explore models of structures that may be otherwise inaccessible to a user. The problem with publishing user-generated models is the difficulty in validating the quality and accuracy of such 3-D models. The quality and accuracy shortcomings are related to both the texture and geometry of the model.
The quality of user generated texture that may be provided to a user-generated 3-D model can be determined by comparing the texture provided the 3-D model with an image or photograph of the structure that is modeled by the 3-D model. However, the geometric accuracy of such a user-generated 3-D model cannot be easily calculated. In particular, if another 3-D model is not available to compare to the user-generated model, it is not feasible to determine the accuracy of the user-generated model's geometry.
Moreover, if a base model does exist for comparison, it is neither easy nor computationally efficient to compare two disparate 3-D models of the same structure. This is because comparing two models that have different representations of mesh geometry and texture does not always yield an accurate similarity measure. It should be appreciated that the accuracy of 3-D models depends on basic factors such as: model geometry, texture (the basic matte color or albido of the object) and material properties (shininess, view dependent texture). Estimating the accuracy of a user-generated model's geometry is difficult because it has to be compared with an existing geometrical representation of the structure's surface (which may not be available) and because comparing two disparate geometries can be difficult.
It should be appreciated that the accuracy of a 3-D model's texture depends on the accuracy of the underlying geometry of the 3-D model as well as the accuracy of its representation of the structure's surface-albedo and reflectance. Therefore, accuracy of underlying geometry of the 3-D model is critical to the overall accuracy and quality of a 3-D model.
Any 3-D model validation approach that directly uses textured models for quantifying model accuracy will depend on the user-generated texture. The texture of a user-textured 3-D model depends on various factors: ambient illumination, level of geometrical detail, texture map resolution, surface reflectance, chromatic variations etc.
Additionally, user-contributed texture depends on the image used for texture mapping the model. The image-pixels corresponding to the texture depend on the viewing angle and internal characteristics of the camera used for generating the image. Thus, a 3-D model validation and verification scheme that is based on user-contributed texture will not be reliable and consistent across all models (the models can have viewing angles that are different from the image). Moreover, not all user-generated models will have textures associated with the model geometry. The above cited challenges make it difficult to measure the accuracy of user generated 3-D models for any verification and validation scheme.
Accordingly, the validation of user generated 3-D models has inherent difficulties related to both texture and geometry. Conventional systems do not overcome these difficulties. The user-generated models of many conventional systems are not validated at all. These models are generally published as presented. Consequently, because they possess inadequate validation processes to ensure the accuracy of user generated 3-D models, conventional systems oftentimes publish unreliable user generated 3-D models.